Update README.md

node-steam now passes an array of cookies
Keep track of the number of pending requests
- Prevents possible duplicate 'end' events etc.
- No need to check for tradePartnerSteamID anymore
Fix _requestsInFlight not decrementing on error
Merge branch 'requestsInFlight' of https://github.com/seishun/node-steam-trade into requestsInFlight

Conflicts:
	index.js